The present invention relates to a method for producing electrochromic films on glass or equivalent substrates, such as ceramics.
Electrochromic films undergo reversible coloration induced by an applied electric field or current. These solid-state inorganic electrochromic layers can be broadly classified into those that color cathodically due to dual injection of electrons and ions (typically group VI-b oxides, such as WO.sub.3 and MoO.sub.3) and those that color anodically due to dual ejection of electron and cation or electron ejection/anion injection (typically group VIII oxides, such as IrO.sub.2, Rh.sub.2 O.sub.3, NiO and CoO). Such coatings are used in information display devices, solar control windows, and light modulators.
Vacuum techniques, typically evaporation and sputtering, are widely used to deposit electrochromic thin films. Nonvacuum techniques, such as anodization and atmospheric chemical vapor deposition are also reported. Evaporation deposition and sputter coating require a high vacuum. The necessary equipment is expensive, making such processes capital intensive. However, they have been commonly used to produce electrochromic coatings.
Three similar nonvacuum coating techniques which have not been used to any significant degree for electrochromic coatings are dip coating, spray coating and spin coating. Dip coating is commonly used to coat glass with SiO.sub.2. It involves lowering a glass substrate into a solution containing an appropriate precursor of the desired oxide..sup.1 This is typically a solution of a partially hydrolyzed alkoxide dispersed in a suitable organic solvent, e.g., tetraethylorthosilicate dissolved in an ethanolic solution and partially hydrolyzed. After dipping the substrate into the solution, the glass is withdrawn at a controlled rate. As the glass is withdrawn (typically at a rate of several centimeters per minute), the solution coats the surface. The coating is then dried and fired in an oven to complete hydrolysis and condensation and to densify the newly formed oxide coating. FNT Spin coating and spray coating are similar except that instead of dipping the glass, the precursor solution is applied to the glass, which is spun to spread the coating out, or is sprayed onto heated glass.
However, the alkoxylates of some of the important electrochromic metal oxides do not afford satisfactory coating results if directly dissolved in typical solvents. For example, unsatisfactory results have been obtained attempting to dip coat a glass substrate in a solution formed by dissolving tungsten butyrate in alcohol solution. Hence, an important low cost approach to creating electrochromic coatings appears to be impractical based on present technology.
French Pat. No. 2,527,219 discloses dipping glass substrates in a colloidal polymetallic acid of a transition metal, preferably in aqueous medium. However, such suspensions are reported to be very unstable, having a useful life of 24 hours or less.
Nakatani et al U.S. Pat. No. 4,420,500 discloses the deposition of transparent conducting films onto glass and ceramic substrates from a coating composition containing indium compounds and alkyl tin nitrate compounds.
Washo U.S. Pat. No. 4,347,265 discloses forming an electrochromic coating gel by dissolving tungsten chloride or molybdenum chloride in an alcohol or other organic solvent, such as xylene. A glass or indium-tin oxide (ITO) coated glass substrate is coated with this coating and then fired to yield a tungsten oxide coating. However, that process is inoperable with a very important electrochromic material, nickel. Mere dissolution of nickel chloride in alcohol does not result in effective electrochromic coating solutions or resultant effective fired coatings.